One of the challenges of image-guided medical and surgical procedures is to efficiently use the information provided by the many imaging techniques the patient may have been through before and during the intervention.
In cardiology, for example the physician often has access to real-time x-ray images acquired by a C-arm. These images have a very good spatial and temporal accuracy enable to follow precisely the progression of thin catheters and other interventional tools. However, soft-tissues are barely visible in these images, and furthermore, these images are projections which do not give a direct access to the volumetric geometry of the intervention scene. To gain access to this important information, a solution consists in using a second imaging modality which is both 3D and able to image soft-tissues.
One possible choice for this second imaging system is 3D ultrasound imaging. The advantage of this modality is that it can be used in real-time during the surgical procedure. In cardiological procedure, trans-esophageal probes can be navigated right next to the heart, producing real-time volumetric images with anatomical details that are hardly visible with standard transthoracic ultrasound.
Typical interventions currently involving this modality combination are ablation for atrial fibrillation, PFO closure (or other septal default repair), and percutaneous valve repair (PVR). All those interventions are x-ray centric, but in all of them, the simultaneous involvement of ultrasound is either very helpful or completely mandatory to monitor the placement of the tool/endoprosthesis with respect to the soft-tissue anatomy.
Although the ultrasound probe can deliver very useful images of the anatomy, an important drawback is the compromise that exists between the temporal acquisition frame rate and the extent of the field of view. It is therefore necessary to have a small field of view to acquire images at high frame rate.
But it is often difficult to select the optimum field of view, which size is constraint by the acquisition frame rate but which at the same time should include the area to be visualized.
Generally, a volume with a large field of view is first acquired and is used to select small sub-regions within this first acquisition corresponding to the area of interest. In many interventions, the area of interest would include the interventional tools or some of them. So in practice, the acquisition volume could be targeted around the interventional tools. Unfortunately, the interventional tools cannot be easily visualized in ultrasound due to artifacts (acoustic reflections, shadows, etc.) and limited spatial resolution.
As a consequence, the actual steering of the probe beam so that it encompasses the interventional instrument is uneasy and requires specialized skill and attention. And this is made worse in interventions where both the anatomy and the device undergo strong movements (atrial fibrillation ablation, PFO closure, PVR).
Ultrasound through x-ray registration is usually performed using image-based registration techniques aiming at lining common structures visualized by both modalities. This approach has several drawbacks.
An important one is the difficulty to include the registration landmarks in the field of view which can be very limited in trans-esophageal echocardiograms (TEE). Moreover, natural landmarks such as the heart contours cannot be used because they are not visible in x-ray. The use of interventional tools as registration landmarks is challenging as they are not well defined in the ultrasound volume due to noise and artifacts.
Ultrasound to x-ray registration can also be achieved using tracking systems which give the position of the ultrasound probe with respect to the x-ray imaging system. Unfortunately, the ultrasound probe does not come with a standard tracking system that could be attached to the x-ray imaging system. Many systems have been designed to gap that void using physical trackers such as magnetic devices. These systems may be expensive and have several disadvantages: they can be disrupted by interference and require additional calibration steps which are prone to error.